The present invention concerns broad band antennas and relates more particularly to antennas adapted to ultra-short high voltage pulses.
All of the broad band antennas at present available on the market are arranged to function in permanent harmonic operation and are used for various applications such as, for example, electromagnetic compatibility tests or measurements of Surface Equivalent Radar or SER. The most widely used are, among others:
horns with baffles
Log-periodic,
Vivaldi antennas,
butterfly antennas,
spirals,
biconical antennas, etc.
In spite of the great diversity of these types of antennas, the majority of them do not offer the features desired for experiments in the transient field.
In order to have a high performance with respect to time, the antennas should be naturally broad band in order to cover the spectrum range of the pulse delivered by an associated pulsed generator. They should in addition have particular qualities, appropriate to the radiation or the measurement of ultra-short pulses. It is in fact important that the antennas have a transfer function with low dispersion in frequency so that the radiated or received pulse is neither deformed nor spread. A significant distortion of the signal results in elongation of the responses with respect to time of the various targets and causes the loss of one of the main interests of the transient methods, that is to say, the possibility of separating the useful echoes from the interference paths by simple time xe2x80x9cwindowingxe2x80x9d.
Among the conventional broad band aerials at present available on the market, horns, horns with baffles and Log-periodic are the antennas most commonly used.
In the following there is given, for each of these types of antennas, the electric field radiated in the axis, when the excitation signal applied to the antenna is a Gaussian pulse, having a width at mid-height of 700 ps.
a) The horn proposed by way of example is designed by means of the code of calculation by finite differences in the transient field. The dimensions of the horn are determined so that its passband extends from 100 MHz to 1 GHz. The excitation of the guide is effected by imposing in a sectional plane a spatial distribution of the electric field according to the TEO1 mode (sinxcfx80y/a) with a: dimension of the guide along the axis y. The pulse radiated in the axis at long range has a time spread of about 80 ns; it is only really significant over 30 ns.
This type of antenna is not therefore adapted to function in transient operation. Each spectrum component is in fact emitted from a phase centre which moves within the horn, thereby in part bringing about the spread of the signal.
Moreover, the size of the antenna at these frequencies becomes very large, hence a not inconsiderable bulk and difficulties of implementation.
b) The horn with baffles has the particular feature of having a large passband (200 MHz-2 GHz) while maintaining relatively modest dimensions. The use of baffles at the exponential profile makes it possible to obtain a high gain over the whole passband. This horn was tested in an anechoic chamber at CELAR. The radiated electric field has a time spread of about 15 ns.
The pulse is in part deformed by the poor performances of the horn at low frequency. Evanescent modes are in fact excited below the cut-off frequency of the guide, thereby disturbing the radiated electric field. The baffles and the reflections at the ends of the plates may also contribute to the dispersion of the signal.
c) The Log-periodic antenna is an assembly of parallel dipoles powered by a transmission line, in such a manner that two consecutive dipoles are of opposite phase.
Each strand radiates with a maximum efficiency when the supply half wave-length is equal to its own length.
Thus, the high frequency of the antenna is limited by the dimension of the smallest strand, and the low frequency by that of the largest strand. The Log-periodic antenna has been designed by means of the code of calculation of integral equations.
The geometrical dimensions were determined so that the antenna should be directional and cover a spectrum of 100 MHz to 1 GHz. This type of antenna mainly emits a horizontal electric field, the duration of which is relatively long.
The successive resonances of the strands constituting the antenna are the origin of the dispersion that can be observed on the radiated signal.
The conventional broad band antennas are not therefore suitable for radiating an ultra-short pulse. Numerous researches have however been carried out for many years in order to design devices capable of radiating high level pulses with a minimum of distortion, but these antennas are not at present available on the market.
It therefore appeared necessary to design an antenna, simple to implement and not bulky, and above all guaranteeing correct electromagnetic performances for the two modes of operation, transient and harmonic.
The subject of the invention is a broad band antenna, characterised in that it comprises in a common plane two symmetrical parts each including at least two interconnected conductor strands powered by a double-wire line, each strand comprising in its portion opposed to the double-wire line a resistive load.
According to other features of the invention:
each symmetrical part further comprises at least one strand not connected to the other strands and including in its portion opposed to the supply line a resistive load,
each symmetrical part comprises n conductor strands, interconnected or not, and each including a resistive load at its end, n being greater than 2.